It is well known that an appreciable impairment in the service life of rotors of thermally loaded turboengines, here, but not exclusively, a steam turbine, originates from the high temperature gradients within the rotor material, specifically especially on the turbine inlet. The high temperature gradients are caused by sudden changes in the thermodynamic conditions during transition phases of the turbine of such a turboengine (for example, during start-up or during a shutdown). During start-up, for example, the rotor is still at a low temperature, whereas the working gas, that is to say the steam in the case of a steam turbine, flows into the hot steam duct with high pressure and high temperature. The rotor surface directly exposed to the hot steam is then brought to higher temperatures, whereas the main part of the rotor body is still at the (low) initial value.
This gives rise to a high temperature gradient between the body and surface, which is converted into mechanical stresses. On account of the incessant start-up and shut-down phases of such a steam turbine, especially in modern quick-start combined-cycle power station applications and in turbines with high steam temperatures (Ultra Super Critical USC), the service life of the rotor is reduced due to the cyclic heat stresses (Low Cycle Fatigue LCF). A reliable algorithm for calculating the remaining service life based on the stress in the rotor is therefore dependent on an exact measurement of the temperature in the rotor inlet region.
Hitherto, the rotor temperature has not been measured directly in the inlet region of the turbine. Instead, for example, the temperature has been measured at various points of the inner casing by means of thermoelements, and the corresponding temperature on the rotor has then been determined from this on the basis of a transfer function between the rotor and casing. On the basis of these measurements, the stress in the rotor and, from this, the remaining service life have then been derived. However, such a procedure has certain limits for rapid transient processes, specifically especially for machines which operate at higher than conventional steam temperatures. In this case, account must be taken of the fact that, for example, an excess of 10% in the mechanical stress of the rotor (in combined-cycle power stations with two shifts) may signify a reduction in the service life of 40%.
U.S. Pat. No. 4,796,465 discloses a method and a device for monitoring the material of a turboengine, in particular of a steam turbine, in which material samples are taken from the forgings of the rotor disks or of other turbine parts and, after the final machining of the forgings, are inserted into recesses provided for this purpose. The samples are then exposed, during operation, to the conditions prevailing there. After a predetermined operating time, the samples are removed again and examined for material fatigue or the like, so that the remaining service life of the machine can be determined. This method is highly complicated and is not very flexible in practical terms.
JP-A-6200701 discloses a method for determining the remaining service life of a rotor of a steam turbine, in which the hardness of a high-temperature part of a new rotor is measured at periodic intervals. From this a hardness reduction rate is calculated, from which the service life of the rotor is ultimately derived. This method also requires access to the stationary machine and is therefore complicated and inflexible.
JP-A-7217407 discloses a method and a device for monitoring the service life consumption of a turbine, in which the surface temperature on a casing and on an intermediate portion of the casing thickness is measured, and the thermal stresses are calculated from the difference and compared with calculated limit values. The method is suitable primarily for static components (casings, valves, etc.). This measurement, at most, makes it possible indirectly to draw conclusions as to the remaining service life of the rotor.
JP-A-63117102 discloses a method for determining the service life of a steam turbine in a bore of the rotor, the electrical resistance in a high-temperature part and a low-temperature part of the rotor being measured by means of an electrical resistance sensor displaceable in the bore. The service life of the high-temperature part is then deduced from the difference in the resistances. This difference measurement requires a complicated built-in movement mechanism which is complicated and susceptible to faults during operation and requires considerable additional costs for building it in and for maintenance.